The present invention relates to the transmission of a keying signal for demodulation based on the gated sync suppression principle of a scrambling system such as used in CATV systems and satellite communication systems.
In CATV systems and direct satellite broadcasting (DBS) systems, a video signal of the pay-TV program is scrambled so that the pay-TV program can be viewed only by subscribers. Numerous methods for scrambling the video signal have hitherto been proposed, but the scrambling method based on the principle of gated sync suppression is currently widely used. According to the gated sync suppression principle of scrambling, the level of a video carrier wave including a color burst during each horizontal flyback period is suppressed 6 dB so that, when the video carrier wave is received by a usual television receiver set, a sync separator in the television receiver (except for that owned by an authorized subscriber) is unable to properly separate the synchronizing pulses from the video signal thereby to rendering the reproduction of the picture carried by the video signal unviewable. The manner by which the video signal is scrambled according to the gated sync suppression principle is shown in FIG. 1, while a scrambler circuit for scrambling the video signal is shown in FIG. 2. Referring first to FIG. 1, the waveform (a) represents a video signal, the waveform (b) represents a horizontal flyback signal included in the video signal (a), the waveform (c) represents the picture carrier wave modulated by the video signal (a), the waveform (d) represents the picture carrier wave modulated by the video signal (a), which is in a scrambled form, i.e., in which the horizontal flyback period is suppressed 6 dB, and the waveform (e) represents a frequency-modulated sound carrier wave superimposed with a keying signal used to restore the signal (d) to the original waveform.
In order to obtain the signals (a) and (e), referring to FIG. 2, when the signal (a) shown in FIG. 1 is applied from a video signal input 201, a synchronizing signal is separated in a sync separator 202. At the same time, the signal from the video signal input 201, after having been delayed a predetermined time by a delay circuit 203 and after having subsequently DC-clamped in a clamp circuit 204 by a clamp pulse 205 fed from the sync separator 202, is modulated at an intermediate frequency by a video modulator 206. An output signal from the video modulator 206 is fed to a sync suppressor 207 at which only the horizontal flyback period is suppressed 6 dB. A gated suppressing signal 210 generated from a keying signal generator 209 in synchronism with a separated synchronizing signal 208, that is, a signal corresponding to the waveform (b) shown in FIG. 1, is applied to the sync suppressor 207, and accordingly, a synchronizing portion is suppressed in response to such signal. On the other hand, a sound signal applied to a sound signal input 211, after having been frequency-modulated in an FM modulator 212, is amplutide-modulated in an AM modulator 214 by a keying signal 213 fed from the keying signal generator 209. Accordingly, an output from the AM modulator 214 represents such a double modulated waveform as shown by (e) in FIG. 1. These two outputs, after having been combined together, are converted by a frequency converter 215 to a desired frequency, and are then outputed through a BPF circuit 216 as an RF outputs 217. The waveforms of the respective RF output 217 are shown by (d) and (e) in FIG. 1, respectively. As can be understood from FIG. 1, since the sound carrier wave is amplutide-modulated, a phase displacement takes place during the process thereof and mixes in the frequency-modulated wave, resulting in that the sound signal subsequently restored contains buzzing noises. In addition, in the case of TV multiplex sound broadcasting, since the keying signal has a frequency component equal to the horizontal period, buzzing noises tend to be reproduced during descrambling.